Trench schottky diodes are known. FIG. 1 shows the cross-section of a portion of a trench schottky which is described in the copending U.S. patent application Ser. No. 10/193,783 assigned to the assignee of the present application.
The trench schottky diode shown in FIG. 1 is formed in an epitaxial silicon layer 10 of one conductivity which is formed over a silicon substrate 12 of the same conductivity. Epitaxial layer 10 includes a lower concentration of dopants than substrate 12. Typically, the epitaxial layer 10 and substrate 12 are doped with N-type dopants.
The trench schottky diode shown in FIG. 1 includes a plurality of spaced trenches 14 which extend from the top surface of epitaxial layer 10 to a predetermined depth. Each trench 14 is lined with an oxide layer 16 at its bottom and sidewalls of a substantially uniform thickness of typically about 500 Å to 750 Å, and includes in its interior an electrode 18 formed of a conductive material such as doped polysilicon.
As seen in FIG. 1, between each pair of trenches 14 a mesa 20 is formed. A schottky barrier layer 22 is formed such that it is in schottky contact with mesas 20 and electrical contact with electrodes 18 inside trenches 14. Schottky barrier layer 22 may be formed from, for example, titanium or titanium tungsten. An anode contact 24 which is preferably formed from aluminum is formed over schottky barrier layer 22. The trench schottky diode shown by FIG. 1 also includes cathode contact 26 formed over substrate 12. Cathode contact 26 may be a solderable contact structure such as a trimetal structure.
The trench schottky diode shown by FIG. 1 also includes termination trench 28 which surrounds the active region of the device. The sidewalls and the bottom of termination trench 28 are also lined with an oxide layer 16. Anode contact 24 extends over the inner sidewall of termination trench 28 and to a portion of the bottom thereof. Also formed at the inner and outer sidewalls of termination trench 28 are conductive polysilicon walls 30. Anode contact 24 may be capacitively connected with epitaxial layer 10 adjacent the inner sidewall of termination trench 28 through polysilicon wall 30. The outer boundary of the trench schottky diode is shown by scribe line 32. FIG. 1 also shows typical dimensional values for the prior art trench schottky diode.
In a device shown by FIG. 1, when oxide layer 16 is kept at or near 500 Å the device exhibits desirable reverse voltage blocking characteristics for a voltage rating of 15 to 20V. However, thin oxide layers are more prone to oxide breakdown under high reverse voltage conditions and to mechanical fracture of points of high stress, like at the edge of metal field plate. To remedy the problem oxide layer 16 can be thickened to up to about 1200 Å. A trench schottky diode with a thicker oxide layer 16, however, exhibits a higher leakage current, which is undesirable.